This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for controlling dispensing of marking particles into a developer unit.
In a typical electrophotographic printing process, a photoconductive member is sensitized by charging its surface to a substantially uniform potential. The charged portion of the photoconductive member is exposed to light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge in the irradiated areas to record an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
It is generally well known that the density or concentration of toner particles has to be maintained within an appropriate range in order to continuously obtain copies having a desired density. However, toner particles are being continuously depleted from the developer material as copies are being formed. Many types of systems have been developed for detecting the concentration of toner particles in the developer material. For example, a test patch recorded on the photoconductive surface is developed to form a solid area of developer material. Generally, the density of the developer material developed on the test patch is monitored by an infrared densitometer. The density of the developed test patch, as measured by the infrared densitometer, is compared to a reference level. The resulting error is detected by a control system that regulates the dispensing of toner particles from a storage container.
However, such a system used to replenish toner particles into the developer material is fairly inaccurate since the infrared densitometer is remote from the developer system. As a result, the amount of toner particles actually dispensed fluctuates around the average value set by the control system. Accordingly, accurate toner particle concentration will not reduce the control bandwidth. One of the major causes of the wide control bandwidth is the delay built into the control loop. The control loop detects low toner particle concentration after this condition has been reached and does not anticipate the requirement to furnish additional toner particlesbefore the low toner particle concentration condition is reached. In addition, added toner particles have to be mixed with the developer material and charged to the appropriate level. Mixing and charging of the toner particles requires time in addition to time required to develop the test patch.
It is also known to divert developer material to a hopper in which a toner concentration sensor is mounted. Such hopper designs, however, require additional space for the hopper and for transporting developer to the hopper, such that the size, cost, material flow time, and torque of the developer assembly are increased.
It is further known that a toner concentration sensor device, comprised in part, of a transformer with a magnetic core, can be mounted in the developer material. Such a device must be placed in the active developer material flow so as to measure the developer material which is actively transported to the developer roll. However, developer transported by augers is prone to shear at the outer most edge of the auger blade, especially at high humidities, so as to not freely flow over the surface of the toner concentration sensor. This results in a sensing inaccuracy and subsequent wide and biased toner concentration control band.